Coupled structural and magnetic properties of ferric fluoride nanostructures part I: A Metropolis atomistic study

Abstract

a b s t r a c tA modified Metropolis atomistic simulation is proposed to model the structure of grain boundaries (GBs) and interfaces in ionic nanostructured systems and is applied to the magnetically interesting case of iron trifluoride (FeF 3 ). We chose long-range interatomic potentials adjusted on experimental results and adapted a previously established Monte Carlo scheme consisting of various modifications of the simulated annealing/Metropolis algorithm. Atomic structures of twisted and tilted GBs as a function of the relative disorientation of the grains have been achieved yielding close to experimentally measured properties. This approach takes into account the structure of the grains far from the interface in order to constrain the relative orientation of the grains, without any periodic boundary conditions. One concludes that a long-range Coulombic fall off of the interatomic potentials is necessary to obtain GB structures presenting a correct local topology but with a smooth transition from crystalline to amorphous states. The structural features are finally discussed in terms of topological aspects and local magnetic structure.

Coupled structural and magnetic properties of ferric fluoride nanostructures: Part II, a Monte Carlo–Heisenberg study

“…In terms of ring statistics, it has been numerically established [23] that all odd rings are localized in the grain boundary of nanostructures, confirming the speromagnetic structure evidenced from in-field Mössbauer spectrometry. The Monte Carlo-Metropolis method has been successfully applied to study the surface and finite-size effects in nanostructures [24][25][26].…”

“…1(a). A comparison with the distribution of the superexchange angle in nanostructured FeF 3 obtained from previous studies [23] implies that all interactions in the GB remain antiferromagnetic. This is observed in Fig.…”

“…Those samples are obtained by Voronoï tessellation and structurally relaxed with a scheme based on a modified Metropolis algorithm [23]. It has been shown both experimentally and numerically in such a system that the grain boundaries are structurally disordered, but still remain composed of cornersharing octahedral units with odd and even iron rings, thus leading to magnetic frustration.…”

“…All samples were first structurally relaxed, with sizes ranging from 8 to 12 nm and different orientations of grains (tilted, nontilted and twisted as in [23]). A simple definition of those terms is given as follows:…”

“…Recent experimental and theoretical studies on the properties of FeF 3 nanostructures obtained by mechanical milling have shown that, at the nanoscale, this material is composed of two parts: the grain which behaves as the crystalline rÀFeF 3 phase and a disordered grain boundary which remains composed of corner-sharing octahedral units, respectively [22,23]. In terms of ring statistics, it has been numerically established [23] that all odd rings are localized in the grain boundary of nanostructures, confirming the speromagnetic structure evidenced from in-field Mössbauer spectrometry.…”